This invention relates to an improvement of the technology to measure an internal signal waveform of a semiconductor device by using a beam of charged particles, and more particularly to an improvement of a charged particle beam device suitable for measuring an internal signal waveform of a semiconductor device which deals with logic signals.
In the Japanese Patent Laid-Open No. 60-127648, there is described a method to measure effectively a logic signal waveform in a semiconductor logic device by using an electron beam. A method of irradiating two or more times (n times) a primary electron beam of pulse form within a period (a logical period) of a logic signal that should be measured is described. In this method, the time needed to measure one logic signal waveform can be reduced to 1/n, compared with the conventional method, in which one pulsed electron beam is irradiated within a logic period. Therefore, it is especially advantageous when the logic signal waveform to be measured has a long period (logic period).
However, in the above described patent publication, neither means nor method is found for processing an output signal generated from a secondary electron detector, which detects a secondary electron radiated from the surface of a specimen or a semiconductor device when the above described, pulsed primary electron beam is irradiated.
The present invention relates to a concrete method for processing the above described, secondary electron detecting signal (pulsed signal), and to means for practicing the method, and more particularly to the method and means which become effective when the pulse width and pulse interval of the secondary electron detecting signal pulse are short.
FIG. 4 shows an example of the concrete construction of the measuring device, which irradiates a primary, pulsed electron beam two or more times within one logic period of a logic signal waveform to be measured, and which is disclosed by the above described publication No. 60-127648. In FIG. 4, a primary electron beam 2 radiated from an electron gun 1, is collected by an electronic lens 6 and focused on a specimen 10 to be measured. This collected electron beam 2 is rotated by a scanning coil 8, and is scanned on the specimen 10 to be measured, in the two-dimensional process. When the primary electron beam 2 irradiates the specimen 10, a secondary electron or a reflected electron is emitted from the irradiated position. The secondary electron or the reflected electron is detected by a detector 9, and, by using a detected signal, an image of the specimen (an image of the secondary electron or the reflected electron) is displayed on the screen of a displaying apparatus 7, which is scanned in synchronism with the scanning of the secondary electron beam 2 by using the scanning coil 8. This is the principle of a device which is called a scanning electron microscope.
The scanning electron microscope can also be applied to observe a surface condition of a semiconductor circuit which is being operated, and to measure an operating signal waveform. In this case, an electrostatic deflection plate 3 is supplied with an output pulse signal from a deflection pulse generating circuit 12, which generates pulsed deflection signals synchronizing with a period of operating signals of a semiconductor circuit or a specimen, and thus the primary electron beam 2 is deflected and pulsed. Only when the primary electron beam 2 is deflected to pass through an opening of a perforated plate 4, can it reach the underside of the perforated plate 4; thus, the primary electron beam 2 is pulsed. If the specimen 10 is scanned by using such a pulsed, primary electron beam 2, which irradiates the the specimen 10 only when its constant phase is kept on the specimen 10, a condition of the specimen of different phase can neither be overlapped nor displayed within its periodical change, and therefore only the condition of the specimen of a certain phase point can be selectively displayed within its periodical change.
In order to observe the whole periodical change on the specimen 10 by using the above described method, a changeable delay circuit 5 is inserted between a deflection pulse generating circuit 12 and a driving circuit 11 to drive the specimen (semiconductor circuit), and, by using the changeable delay circuit 5, the timing to generate a deflection pulse signal is made to deflect little by little, comparing to the timing for generating a clock signal to drive the specimen. The specimen to be observed and measured by using the device is preferably a semiconductor integrated circuit (LSI). In this case, based on the timing to generate the clock signal to drive the LSI or specimen, irradiating timing of the pulsed, primary electron beam 2 is determined. And, the timing to generate the pulsed, primary electron beam 2 should be shifted little by little within at least a period (T) of the internal operating signal waveform of LSI. For this purpose, the above described changeable delay circuit 5 is arranged. Therefore, the maximum required delay time (.tau.) of the changeable delay circuit 5, should at least be equal to a period (T) of the internal operating signal waveform of the specimen (LSI).
When a period (t) of a clock signal to operate the specimen (LSI) coincides with the period (T) of the internal operating signal waveform to be observed and measured, the maximum delay time (.tau.) of the changeable delay circuit 5 can be equal to the period (t) of the clock signal. However, if the period (T) of the internal operating signal waveform to be observed and measured becomes n times the clock signal period (namely T=n.multidot.t), the maximum delay time (.tau.) of the changeable delay circuit 5 should be n times the clock signal period (.tau.=T=n.multidot.t), too; and therefore the time required to observe and measure the internal operating signal, increases n times.
Thus, in the conventional technology shown in FIG. 4, even if the period (T) of the internal operating signal waveform of the specimen (LSI) to be observed and the measured is n times the clock signal period (t), the pulsed primary electron beam 2 is made to generate once within a period (t) of the clock signal which operates the specimen (LSI), always in synchronism with the clock signal; and then each pulsed primary electron beam is assigned to measure 1/n of the required measuring range (period (T)) of the internal operating signal waveform that should be observed and measured. And, in this measuring system, a gate circuit 18 having n gates G1 to Gn is provided for opening and closing, in synchronism with the delayed clock signal, and for processing a signal detected by the secondary electron (reflected electron) detector 9; and, after the detected signal is passed through each gate, it is stored in memory circuit 23, which has a plurality of memory cells M1 to Mn. That is, the detected signal from the detector 9 is once stored in an individual memory cell by each clock period, and then the stored signal is read out from the memory cells one by one so as to be synthesized as a detected signal for the whole measuring range (period (T)) of the internal operating signal waveform to be observed and measured. The synthesized, detected signal is stored in a memory circuit 24. By using the synthesized, detected signal, the whole of the internal operating signal waveform to be observed and measured can be displayed. By the way, in FIG. 4, numeral 15 represents a control circuit to make open or closed a plurality of gates of the gate circuit 18.
A technical problem of the above described measuring system is a following speed of the gate circuit 18 when the clock frequency becomes high accompanied by the specimen or LSI becoming high speed. When using a clock signal of low frequency, for example 1 MHz or so, for opening and closing one by one a plurality of gates of the gate circuit 18, each detected signal pulse which is taken in by using the gate circuit 18 separately for each clock period, can be integrated by an integrating circuit, and then sampled. However, when the clock frequency becomes high, for example 50 MHz, not only the following speed of the above described integrating circuit but the speed of the gate circuit will have a problem.
As described above, when measuring an internal signal waveform having a clock frequency of 50 MHz in a high speed device by using the conventional technology, it has not been sufficiently considered to process an output signal (pulsed signal) generated from the detector.